RF-based linear charged particle accelerators typically operate with beam pulses timed to coincide or nearly coincide with the crest of the accelerating waveform so as to utilize the full available RF gradient and maximize the output energy. As a result, energy recovering linear accelerators decelerate the beam at or near the trough of the RF waveform. This is normally done fully out of phase with the accelerated beam so that the RF power drawn by the acceleration process is fully replaced by RF power drawn from the recovered beam.
This process cannot always be implemented in the event that the beam energy spread enlarges during transport from acceleration to deceleration. If some process (such as extraction of power from the beam using a Free Electron Laser, the quantum excitation of the beam energy spread due to synchrotron radiation processes, the enlargement of the beam energy spread due to use of the beam in a charged particle beam cooling system, or any other process leading to coherent or incoherent growth in the beam energy spread) enlarges the beam energy spread before the start of energy recovery, the highest energy components of the beam energy spectrum can in fact instead reside at energies higher than the available decelerating gradients can recover. This is due to the proximity in time of the beam to the trough of the RF waveform. As a result, after energy recovery the beam will have an extremely large relative momentum spread with attendant operational difficulties (such as severe beam loss). This is schematically illustrated in FIG. 1 which denotes the prior art methodology of energy recovery and wherein Df (as defined below) equals 180°.
There thus exists a need for an energy recovery system for an efficient energy recovery system that does not concurrently impose an extremely large relative momentum spread with the attendant operational difficulties in RF-based linear charged particle accelerators.